Although overmoded waveguides are generally recognized as undesirable in microwave systems, they are nevertheless often used to minimize losses in many modern microwave systems such as television transmission systems. This use of overmoded waveguides presents a problem, however, in that such waveguides allow the propagation of higher order modes of the desired signal which is typically propagated in the dominant mode, such as the TE.sub.11 mode in circular waveguide systems. The higher-order modes are undesirable because they give rise to a group delay problem. Thus, at the ends of an overmoded section of waveguide, certain of the higher-order modes are reconverted to the desired mode, but only after they have traveled through the overmoded waveguide at different velocities. Because the different modes travel at different velocities, the signals reconverted to the desired mode are not in phase with the original signal in that same mode. This problem becomes more serious as the length of the overmoded waveguide is increased, and in many applications such as television transmission systems the overmoded section of waveguide may be hundreds or thousands of feet in length.
For low power applications, internal absorptive filter elements can be used to alleviate the moding problems. Unfortunately, such filters are impractical for high-power applications such as UHF television transmission systems (which typically operate at power levels of at least 30 kilowatts), as the power absorbed may exceed the power absorption capacity of the filter. Furthermore, it is difficult to raise the capacity of such filters by increasing the size of the filter elements because the size required to filter the undesired-mode signals in high-power applications would necessarily interfere with the desired signals propagating in the dominant mode.